1. Field of the Invention
This invention relates generally to nanoscale energy storage units. It relates particularly to multilayer structures of bioinorganic proteins, especially ferritins, as electrode materials for bio-nanobatteries.
2. Description of Related Art
Miniaturization of electronic devices has been a trend in high technology areas, enhancing the efficiency, reducing the power consumption, and increasing performance, speed and functionality. Since 1954, nanotechnology has been a challenge in science and technology. The development of nano-scale devices has accelerated in recent years. It is deemed that progress in the development of nano-scale devices necessitates micro- or nano-scaled power sources. Although nano-scaled devices are near reality, the development of micro- or nano-scale energy storage technology has not been effectively considered and developed for the benefit of nano-scaled devices. The exploitation of these nano-devices will depend on the development of a suitable power supply. Currently, the power systems for many nano-scaled or even micro-scaled devices are many times larger in size than the functional devices. Nano-scaled energy storage systems are critically needed for nano-devices where a large power supply eliminates the benefits accrued by miniaturizing device size, or where a large power supply makes its application impractical.
The bio-nanobattery concept is based on a nano-scaled power storage element which is made out of bioinorganic proteins, making it suitable for use with autonomous nano-scaled devices. This bio-nanobattery has a number of advantages, including nano-scaled system sizes, flexible array structures, distributed power storage, thin-film fabrication, and potential integration with energy harvesting units.
The capacity of a ferritin-based bio-nanobattery is determined mainly by two factors: the redox potential difference between two electrodes and the number of ferritins per unit area of electrode. The potential difference depends on a selected pair of ferritin electrodes. When electrochemical stability is considered for Fe(OH)2/Fe(OH)3∥Co(OH)3/Co(OH)2 for example, thermodynamic calculation estimates a battery cell potential of 0.66 V from equilibrium potentials of Fe and Co. Other materials carrying a larger reduction potential can produce higher current. If cobalt is replaced with nickel, a similar calculation based on thermodynamic equilibrium potential yields 0.97 V from Fe(OH)2/Fe(OH)3∥Ni(OH)3/Ni(OH)2. The other way to increase the battery capacity is to increase the number of ferritins deposited on a conductive electrode surface such as gold, indium tin oxide (ITO) glass, or doped silicon. Accordingly, multilayered arrays of ferritin proteins should be prepared with a regular and uniform structure to maximize the number density of ferritin in the array structure.
Biological molecules usually form regular nanostructures under certain conditions. Some proteins, for example, enable two- or three-dimensional ordered arrays on a specific substrate. Using such self-assembling characteristics of bioinorganic proteins, the nano-scaled energy storage units can be assembled into well-organized arrays of single or multilayers. Historically, ferritin arrays have been produced in ways such as Langmuir-Blodgett deposition at air/water interfaces, mechanical scratching method, protein crystallization techniques, and physical adsorption, among others.
However, these methods have been found wanting, because they do not provide thin films which are highly ordered uniform, stable and robust, in short periods of time.